Photochromic or phototropic glasses, as such have been variously termed, had their genesis in U.S. Pat. No. 3,208,860. That patent disclosed glasses demonstrating photochromic behavior, i.e., the glasses darken (change color) when exposed to actinic radiation, commonly ultraviolet radiation, and return to their original state when the radiation is removed. Such glasses are produced by developing crystallites of a silver halide selected from the group of AgCl, AgBr, and AgI therein. The preferred base glasses are stated to reside in the alkali aluminoborosilicate composition system.
Subsequent to that fundamental disclosure, intensive continuous research has been carried out, as is evidenced by the great volume of patent literature, to design glass compositions manifesting ever improved photochromic properties. The most extensive commercial application of photochromic glass has been in the field of spectacle lenses, both as prescription lenses and as non-prescription sunglasses. In view of that situation, research has focussed principally on developing glasses which darken to a low luminous transmittance, which fade very rapidly to their original state, and which retain those darkening and fading characteristics over the range of temperatures commonly encountered by the wearer of spectacle lenses; i.e., between about 0.degree.-100.degree. F. (.about.-18.degree. to +40.degree. C.).
In recent years organic, non-photochromic plastic lenses have made substantial inroads into both the prescription and non-prescription lens markets, principally because of the light weight of such lenses. Thus, the density of the plastic employed in such lenses is considerably less than glass used for the same purpose. Plastic lenses also possess the capability of being readily dyed, imparting a permanent tint thereto with consequent reduction in transmittance, but, as of the present date, no commercially practical photochromic lens has been produced from a plastic.
To gain the perceived advantage of lower weight while enjoying the effect of photochromic behavior, laminated lenses have been produced consisting of an element of photochromic glass and one or more elements of plastic. As can be appreciated, to achieve the desired weight advantage, the glass element will be as thin as possible consistent with the objective of achieving both satisfactory photochromic behavior and essential strength levels in the lens design. The conventional ophthalmic lens has a thickness of about 2 mm; to decrease the weight of the lens it would be desirable to substantially reduce this thickness. Nevertheless, as can be appreciated, as the cross section of a glass article is reduced, the transmittance thereof normally increases. Consequently, to be useful in thicknesses less than 2 mm, the composition of the photochromic glass must be designed to darken to a low luminous transmittance in thin cross sections while retaining the characteristics of fast fading and temperature independence.
U.S. Pat. Nos. 4,018,965 and 4,130,437 disclose the production of non-prescription photochromic sunglasses having thicknesses between about 1.3-1.7 mm. The regimes of glass composition operable in each disclosure are tabulated below in terms of weight percent on the oxide basis.
______________________________________ U.S. Pat. No. U.S. Pat. No. 4,018,965 4,130,437 ______________________________________ SiO.sub.2 54-66 54-66 Al.sub.2 O.sub.3 7-15 7-15 B.sub.2 O.sub.3 10-25 10-25 Li.sub.2 O 0.5-4 0.5-4 Na.sub.2 O 3.5-15 3.5-15 K.sub.2 O 0-10 0-10 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 6-16 6-16 PbO 0-3 0-1.25 Ag 0.1-1 0.1-0.3 Cl 0.1-1 0.2-1 Br 0-3 0-0.3 CuO 0.008-0.16 0.002-0.02 F 0-2.5 0.2.5 ______________________________________
The glasses of U.S. Pat. No. 4,018,965 darken at ambient temperatures to a luminous transmittance not exceeding 25%, and exhibit a fading rate such that, after five minutes' fading from the darkened state, the luminous transmittance of the glass is at least 1.5 times that of the darkened transmittance. However, their design does not include compositional constituents needed to obtain refractive index correction to the industry-standard value n.sub.D =1.523.
The glasses of U.S. Pat. No. 4,130,437 darken at ambient temperatures to a luminous transmittance below and demonstrate a fading rate such that, after five minutes' fading at ambient temperature, the glass exhibits a luminous transmittance at least 1.75 times that of the darkened transmittance. Again these glasses are not corrected for refractive index.
U.S. Pat. No. 4,168,339 is drawn to the manufacture of photochromic glass in the form of micro-sheet, i.e., sheet glass having a thickness between about 0.25-0.5 mm, the glass consisting essentially, in weight percent on the oxide basis, of:
______________________________________ SiO.sub.2 54-66 PbO 0.4-1.5 Al.sub.2 O.sub.3 7-16 Br 0.2-0.5 B.sub.2 O.sub.3 10-30 Cl 0.5-1.2 Na.sub.2 O 3-15 F 0.2-0.5 Li.sub.2 O 0-4 CuO 0.008-0.03 K.sub.2 O 0-10 Ag &gt;0.03-1 ______________________________________
With respect to photochromic behavior, the latter glasses darken at 20.degree. C. to a luminous transmittance below 25%, and display a fading rate at 20.degree. C. such that, after five minutes' fading, the transmittance of the glass increases at least 20 percentage units and, after a one-hour fading period, the glass manifests a luminous transmittance in excess of 80%. These glass are, however, not corrected for refractive index and are not of a composition appropriate for the manufacture of structural glass lens elements of more moderate thickness.
U.S. Pat. No. 4,190,451 discloses photochromic glass compositions which encompass prescription lenses marketed by Corning Glass Works, Corning, N.Y., under the trademark PHOTOGRAY EXTRA.RTM.. The claims encompass glasses consisting essentially, in weight percent on the oxide basis, of:
______________________________________ Li.sub.2 O 0-2.5 P.sub.2 O.sub.5 0-25 Na.sub.2 O 0-9 SiO.sub.2 20-65 K.sub.2 O 0-17 CuO 0.004-0.02 Cs.sub.2 O 0-6 Ag 0.15-0.3 Li.sub.2 O + Na.sub.2 O + K.sub.2 O + Cs.sub.2 O 8-20 Cl 0.1-0.25 B.sub.2 O.sub.3 14-23 Br 0.1-0.2 Al.sub.2 O.sub.3 5-25 ______________________________________
wherein the molar ratio alkali metal oxide:B.sub.2 O.sub.3 is in the range 0.55-0.85 and the weight ratio Ag:(Cl+Br) is in the range 0.65-0.95.
The patent specifies photochromic properties in terms of 2 mm thickness whereby, at 20.degree. C., the glass darkens to a luminous transmittance below 40% and fades at least 30 percentage units after five minutes' removal from actinic radiation. The patent further notes that a particular advantage exhibited by these glasses is the relative temperature independence of the photochromic properties. PHOTOGRAY EXTRA.RTM. brand lenses have the approximate composition, in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 55.8 ZrO.sub.2 4.89 Al.sub.2 O.sub.3 6.48 TiO.sub.2 2.17 B.sub.2 O.sub.3 18.0 Ag 0.24 Li.sub.2 O 1.88 CuO 0.011 Na.sub.2 O 4.04 Cl 0.20 K.sub.2 O 5.76 Br 0.13 ______________________________________
wherein Ag, CuO, Cl and Br represent analyzed values.
U.S. Pat. No. 4,358,542 discusses the production of photochromic glasses in sheet form which possess the capability being heat treated to simultaneously induce photochromic behavior therein and sag the glass into molds to form eyeglass blanks of a proper curvature. Those glasses consist essentially, in weight percent on the oxide basis, of:
______________________________________ SiO.sub.2 55-60 PbO 0.1-0.25 Al.sub.2 O.sub.3 9-10 Ag 0.1-0.15 B.sub.2 O.sub.3 19-20.5 Cl 0.3-0.5 Li.sub.2 O 2-2.5 Br 0.05-0.15 Na.sub.2 O 2-3 CuO 0.0065-0.01 K.sub.2 O 6-7 ______________________________________
At a thickness of 1.5 mm these glasses demonstrate photochromic properties such that they exhibit a darkened luminous transmittance at 20.degree. C. below 25% and a fading rate such that, after five minutes' fading at 20.degree. C., a luminous transmittance at least twice that of darkened transmittance is attained. However, the compositions are not designed for applications wherein refractive index correction is required, nor where glass thicknesses substantially below 1.5 mm are to be used.
U.S. Pat. No. 4,374,931 reports photochromic glasses which darken to low transmittance values in very thin cross sections and fade very rapidly. Thus, at a thickness of 0.4 mm, the glasses exhibit a darkened luminous transmittance at 25.degree. C. below 45% and a half fading time of five minutes. These glasses consist essentially, in weight percent on the oxide basis, of:
______________________________________ SiO.sub.2 52-59 TiO.sub.2 0-3 B.sub.2 O.sub.3 18-23 Ag 0.17-0.22 Al.sub.2 O.sub.3 6-8 Br 0.06-0.12 Li.sub.2 O 1-2.5 Cl 0.29-0.35 Na.sub.2 O 1-3 CuO 0.012-0.019 K.sub.2 O 8-13 PbO 0.1-0.15 ZrO.sub.2 2-6 ______________________________________
While the foregoing compositions have been found to exhibit good photochromic darkening in thin cross-section, the fading characteristics of the glasses are somewhat less rapid than would be desirable. Further they do not exhibit truly neutral coloration in the dark state.
U.S. Pat. No. 4,407,966 claims photochromic glasses manifesting very fast fading capabilities consisting essentially, in weight percent on the oxide basis, of:
______________________________________ SiO.sub.2 56-60 PbO 0.1-0.15 Al.sub.2 O.sub.3 6-9 Ag &gt;0.15-0.25 B.sub.2 O.sub.3 18-21 Cl 0.2-0.35 Li.sub.2 O &gt;2.5-3.5 Br 0.075-0.15 Na.sub.2 O 0.5-2.5 CuO 0.004-0.02 K.sub.2 O 5-7 CeO.sub.2 0-0.5 ZrO.sub.2 3.75-5 ______________________________________
At thicknesses of 1.5-2 mm these glasses demonstrate photochromic properties such that they exhibit a darkened luminous transmittance at 25.degree. C. below 25% and a fading rate such that, after five minutes' fading at 25.degree. C., the luminous transmittance increases at least 35 percentage units. These photochromic glasses, however, are not designed for use in applications requiring refractive index correction plus glass thicknesses on the order of 1 mm or less, particularly as regards the need to obtain both a high level of photochromic darkening in thin cross-section and rapid fading. Thus, while the glasses of this patent and of U.S. Pat. No. 4,374,931 above can include constituents such as ZrO.sub.2 and TiO.sub.2 to raise the refractive index, attempts to develop index-corrected versions thereof retaining the desirable photochromic properties have not been commercially successful.
Notwithstanding the extensive effort to optimize photochromic properties for a wide range of optical and ophthalmic applications as evidenced by the above patent, there remains a need for a glass suited specifically for the manufacture of photochromic glass-plastic laminated lenses of a type wherein the glass element is reduced in weight yet remains a major structural and optical component of the lens system. More specifically, a photochromic glass exhibiting photochromic performance properties, in 1 mm thickness, essentially equivalent to those of the available 2 mm-thick photochromic products is needed. In addition to acceptable photochromic performance, however, the glass must exhibit excellent chemical strengthening characteristics, be capable of refractive index matching to commercial ophthalmic standards (n.sub.D =1.523), and exhibit acceptable melting and forming properties.
It is therefore a principal object of the present invention to provide novel compositions for photochromic glasses demonstrating the characteristics necessary for thin ophthalmic lenses.
It is a further object to provide a thin photochromic glass lens element suitable for use in the manufacture of prescription glass-plastic laminated lens products and/or lightweight all-glass lens constructions.
Other objects and advantages of the invention will become apparent from the following description thereof.